Hi,
Can you tell me how you recorded "going" answer? Subscription list?
If so, you can send a follow-up message to the customer via Subscription list.
If not, after testing by button while recording customers seems to be unable to do.
I have another method, please try:
1. Create a new entity to associate Contact and Event into.
2. Create a new field "Attend or not" in this entity.
3. Design a marketing form for customers to fill out to "going" or "Not going".